When a sliding member is produced from a light metal such as an aluminum alloy, a ceramic film is generally formed on the sliding part of the sliding member by anodizing treatment, electroplating or vapor phase epitaxy to impart wear resistance to the sliding member. The anodizing treatment for use in forming a wear-resistant film on a valve metal typified by aluminum is excellent in the throwing power and in the reduced environmental load because of the non-use of chromium and nickel, and is therefore widely adopted.
Of such anodized films, particularly an anodized film having excellent wear resistance is called a hard anodized film. The hard anodized film is generally formed by a low temperature method. The low temperature method involves anodizing in a sulfuric acid-based electrolytic bath at a bath temperature of up to 10° C. In addition, in the low temperature method, the anodizing treatment is performed at a relatively high current density of 3 to 5 A/dm2 compared to other anodization methods. The hard anodized film obtained by the low temperature method typically has a Vickers hardness of 300 to 500 Hv, and is more compact than other anodized films.
Hard anodized films are currently used, for example, in the sliding part of aluminum alloy machine components, and with the increase in the severity of the sliding conditions, further improvement in the wear resistance is awaited. It is difficult to form a hard and compact anodized film on die casting aluminum alloys.
Anode spark discharge methods in which a spark discharge is used to form a film are also known to form a film with a high surface hardness (see, for example, Patent Literatures 1 to 3). In the conventional anode spark discharge methods, alkali metal silicates, alkali metal hydroxides, and oxygen acid catalysts have been used in the electrolysis solution.
Patent Literatures 1 and 3 describe methods of forming a super-hard film containing α-alumina as its main ingredient by the treatment using a voltage as high as at least 600 V. The film obtained by these methods has an extremely high hardness as represented by the Vickers hardness exceeding 1,500 Hv. In addition, while the thickness of the film that can be formed by the anodizing treatment using an ordinary alkaline electrolysis solution is approximately 10 μm, the thickness of the film formed by these methods may be as thick as 100 μm or more. Accordingly, a film having excellent wear resistance and corrosion resistance can be formed by increasing the thickness of the film.
Other anode spark discharge methods have also been disclosed. Patent Literatures 4 to 6 each describe a method which uses an electrolysis solution of substantially the same composition as that in Patent Literature 3 and a special current waveform to form a film on the surface of a substrate more efficiently than in the method described in Patent Literature 3.
Patent Literature 7 describes an anode spark discharge method in which the smoothness, hardness, and film-forming rate have been improved by using a silicate in combination with lithium ion and sodium or potassium ion.
Patent Literature 8 describes a method of electrolytic ceramic coating on metal wherein an electrolytic treatment is performed using the metal as an anode in an electrolysis solution containing a zirconium compound to form a ceramic film on the surface of the metal.
Patent Literature 9 describes a method for coating a metal with a ceramic film comprising the step of causing glow discharge and/or arc discharge on a surface of a metal substrate which is used as a working electrode in an electrolysis solution to electrolytically form the ceramic film on the surface of the metal substrate, wherein the electrolysis solution contains zirconium oxide particles having an average particle size of up to 1 μm in a content X, and a compound other than the zirconium oxide which is a compound of at least one element selected from the group consisting of Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, In, Sn, Ba, La, Hf, Ta, W, Re, Os, Ir, Pt, Au, Bi, Ce, Nd, Gd, and Ac in a content of Y, and the X and the Y satisfy the following relations (1) to (3) and the electrolysis solution has a pH of at least 7.0.0.05 g/L≦X≦500 g/L  (1)0 g/L≦Y≦500 g/L  (2)0≦Y/X≦10  (3)